The monoclonal antibody MH9A4 recognizes interleukin-9 (IL-9), a proinflammatory cytokine historically believed to be involved in type 2 immune responses. Recent evidence suggests IL-9 may instead be secreted by a distinct T-helper lineage called Th9. These cells can be derived from Th2 cells with TGF beta or differentiated directly from naïve CD4+ T cells with TGF beta and IL-4.

IL-9 is a member of the common cytokine receptor gamma chain-dependent family of cytokines which also includes IL-2, IL-4, IL-7, IL-15 and IL-21. Its pleiotropic effects on Th2 lymphocytes, B lymphocytes, mast cells, eosinophils and gut and airway epithelial cells have implicated IL-9 in asthma and other allergy-related disorders. Staining with the MH9A4 antibody is not blocked by the MH9D1 antibody, suggesting that the two antibodies bind to unique epitopes.